Late Thursday night, Israel launched what many viewed as inevitable: a ground assault into the Gaza Strip, escalating the conflict after ten-days straight of bombing the Palestinian territory. As the tanks and warships surrounding the tiny swath of land began to fire, the Israeli government announced that the mission of this operation would be to collapse the network of underground tunnels running beneath Gaza, particularly those that reach into Israel. But the tunnel system in question goes beyond arms running, providing an estimated $700 million to the local economy. Here’s what you need to know about these tunnels:

There are hundreds of tunnels running under the Gaza-Egypt Border.

At present there are hundreds of smuggling tunnels laid underneath of Gaza, mostly crossing between the Egyptian border town of Rafah, split in two as part of the 1979 peace deal between Egypt and Israel, and the Gaza Strip. “At any one time, there are between 10 and 30 main tunnel shafts,” the Congressional Research Service wrote in 2008. “Wealthy families in Rafah … finance tunnel openings to these main shafts in either their private homes or rented properties.” These openings, the CRS explained, are then rented to groups like Hamas, which currently controls the Gaza Strip, or other militias who then move weapons through the passageways and into Palestinian territory.

Literally everything you can imagine passes through these tunnels at this point.

However, it’s not just guns and other illicit materials that run through the tunnel system. Tunnel owners get a cut of what passes through, leaving open the possibility for almost anything to move under the ground. “One operator told of the time he tried to smuggle in a lion for a Gaza zoo,” National Geographic wrote in a story on the tunnels in 2012, telling of one of the more improbable crossings. “The animal was improperly sedated, awoke in the tunnel mid-trip, and tore one of the workers apart.”

All told, what passes through the tunnels makes up a substantial portion, if not the vast majority, of the Gazan economy at this point. In October 2011, United Nations figures estimated that “800,000 liters (around 5,000 barrels) of fuel, 3,000 tons of gravel, 500 tons of steel rods and 3,000 tons of cement” passed through the tunnels daily.

What has Israel even more concerned than the Gaza-Egypt tunnels, however, is the fact that the system extends much further than the eastern border. These tunnels underneath the 141 square mile territory can be used for militants to travel without being surveyed, Israel argues, making them a threat. A small number also stretch all the way into Israeli territory. “At dawn on Thursday, 13 Hamas gunmen from Gaza emerged from the mouth of an underground tunnel about a mile away, inside Israel territory,” the New York Times reported on Friday. “The air force thwarted the attack, but the government said that the attempted incursion was the final straw and that the ground invasion would commence.”

Though the tunnels date back to the 1980s, they’ve became extremely important after Israel began blockading Gaza.

The reason that such a large portion of the Gazan economy is dependent on the tunnels is that most other crossings into the territory are blocked off. Since Hamas took control of the Strip in 2007, Israel has maintained a blockade on Gaza, allowing “the entry only of goods ‘vital for the survival of the civilian population,’ banning exports, and prohibiting Palestinians themselves from leaving the Gaza Strip in all but the most exceptional cases.” In theory, the limitations are to prevent Hamas from importing weapons into Gaza, as well as cutting off building supplies needed to construct more tunnels. In practice, this has only increased the important role the tunnels play in Gaza.

While the tunnel economy has been booming, other methods of circumventing the blockade have been less than successful, to be put mildly. In 2011, a flotilla including a Turkish-flagged ship called the Mavi Marmara attempted to sail into Gaza containing humanitarian assistance for the Palestinian people. In blocking the flotilla’s advance, Israeli sailors opened fire on the Mavi Marmara, killing nine passengers including one U.S. citizen. In response to the international outcry over the incident, Israel loosened the blockade slightly, reducing restrictions on food but maintaining its embargo on the majority of items like fuel and concrete.

Israel has tried to take them out before.

In late 2008, the IDF launched an airstrike against a tunnel in Gaza it claimed was used to conduct operations against Israel. That strike broke the relative calm of the ceasefire from the 2006 war the country had fought against Hamas and Hezbollah in Lebanon. In December, Israel launched “Operation Cast Lead” in response to an increased tempo of rocket attacks from Gaza, much as in the current instance pummeling the territory with airstrikes before launching a ground operation.

Unlike what’s known so far about the latest phase of Operation: Protective Edge, the current assault on Gaza, Cast Lead had broad goals of not only destroying the tunnel system, but also taking and holding territory that Hamas used to launch rockets. That invasion into Gaza lead to the death of at least 1,440 Palestinians, but failed in its overarching goals. “Moreover, while Israeli airstrikes may have destroyed up to 80% of the estimated 300 tunnels under the border, some tunnels still are visibly in use and others are being repaired,” the Congressional Research Service wrote in 2009. Five years later, its clear that the tunnels survived and flourished.

If the rate at which the Israeli Defense Forces are proceeding holds, this will not be a swift operation. On Friday, the IDF’s spokesperson unit announced that it had unearthed eight tunnels on the first day of the ground assault. The longer the incursion, however, the greater the odds that the already high death toll — constituting more than 200 Palestinian lives lost since airstrikes began last week — will grow even further.